Seen this twice in my career. Internal gap at the anode button and internal dag. Got worse when the brightness was turned up. One of them could actually be seen! This was not an ocassional thing, it was steady and wasn't very loud either. This was on 25' rectangular tubes with red internal coating. Replaced under warranty fortunatly for the customer.

Just FYI

The internal dag forms part of the circuit for the CRT beam current. That is why the arc occurs when the brightness is turned up: the voltage drop across the break inreases hence the arc.

I’ve had the CRT out of the cabinet for a thorough cleaning and inspection. The outer dag is tight with little to no damage and the few scratches were repaird before re-installation. All ground straps are present, clean and tight. Not wishing to dismantle another great condition set for a weaker CRT I’ve opted to leave this one in place. All guns are at 100% and the set produces a great picture in a normally lit room with a brightness setting just under the point of arcing so I’ll just have to remember not to turn up the brightness. Thanks for all the tips guys. Each and every one has been investigated and where applicable substitutes have been made with no change in the condition so I’m convinced it is internal. I believe if you were in the room with the set you’d agree. Once I finish the convergence and purity I’ll post a picture. Thanks again everyone, Rick